Generally speaking, solder (a lead-tin alloy or a tin-containing alloy) has a low melting point and is soft. The solder has a certain degree of corrosion and wear resistance and is often applied for soldering joints of electronic components and printed circuit boards. In the fabrication processes of electronic components or printed circuit boards, the solder bumps can be formed by physical vapor deposition, wave soldering and electroplating, etc. The electroplating technology has been widely used and research on this subject has been started early and the research is still developing.
The most commonly used solder electroplating solution is a fluoroborate electroplating bath. A typical fluoroborate solder electroplating solution contains about 465 g/l of zinc fluoroborate, about 67 g/l of tin fluoroborate, about 25 g/l of fluoroborate, about 40 g/l of boric acid and other suitable additives. The solution uses Pb(BF.sub.4).sub.2 and Sn(BF.sub.4).sub.2 as the source of Pb and Sn, respectively. HBF.sub.4 increases the throwing power and stabilizes the solution. H.sub.3 BO.sub.3 inhibits the decomposition of chloroborate. Other additives increase the smoothness and gloss of solder plating, electroplating rate, crystal refining, etc. Interfacial agents, such as betaines, ethylene oxide polymers, quaternary ammonium compounds, amines, amides, alkyl sulfonates, non-ionic amines, cationic amines, and aldehydes, have been developed as an additive that can effectively control the growth of solder plating. However, these interfacial agents are not used to control the relative content of Pb/Sn in the solder plating. Other solder electroplating bathes, besides fluoroborate, include: sulfonic acid bath--wherein acetaldehyde or phenolsulfonic acid is used as the gloss agent; sulfite bath (containing benzaldehyde, carboxylic acid, amide, esters, etc.); aromatic sulfonate bath and pyrophosphate bath, etc.
There are many ways to control the alloy composition of solder plating in the solder electroplating process. The Pb/Sn concentration ratio in the electrolyte is the dominant factor influencing the alloy composition of solder plating. As the content of bi-valence tin increases, the tin content in the solder plating will increase as well. However, in the method of using Pb-Sn concentration to control the solder plating composition, the deposition percentage of metals in the electroplating solution is not high. As a result, the production efficiency is not high and the process is not economical. This is one of the problems that the invention intends to solve.
Generally speaking, the current density will increase the tin content in the solder plating while some additives might also influence the tin content in the solder plating. The composition of the anode used in electroplating is one of the factors affecting the tin content. Common solders used in the electronic industry include 63Sn-37Pb, 95Pb-5Sn, 99Pb-1Sn; wherein 63Sn-37Pb is easier to form solder plating through electroplating process and is commonly used. The melting point of 63Sn-37Pb is the lowest among the solders mentioned. By controlling the above variables, a solder plating with desired alloy composition might be achieved; however, the deposition percentage of Sn is not easy to be increased. As a result, the current efficiency is adversely affected and the electrolyte is not used efficiently.
When the solder electroplating technique is used in the fabrication of electronic components and devices, the mechanical and electrical properties of the solder plating must be taken into consideration. These properties are closely related to the microstructure of solder plating. A. Brenner, in Electrodeposition of Alloys, Academic Press, London, Vol. 1 and Vol. 2, states that when the tin content in the solder plating is about 37 wt %, the grain size of solder plating microstructure is the smallest. Theoretically, a well control of electroplating current may lead to a fine microstructure of solder plating--i.e., a fine surface crystal size. The crystal size of solder plating directly influences its mechanical properties. When the crystal size is relatively small, the strength of the solder plating is relatively high. Therefore, the control of crystal size through simple means is very important to industrial applications. The main purpose of the invention is to provide an improved fluoroborate electroplating solution, thereby one can effectively control the tin content of solder plating and increase the deposition of Sn ions as well as enhance the microstructural fineness of solder plating.